The push will always be to reduce weight, the reality is the only downside to a FF (apart from the slightly more chunky looks) is the weight, as Jeremy Clarkson would say, you can never have too much powerrrrrrrrr ?. Joking aside the reduced power of the SL is a by product of getting the weight down. An ideal bike would be FF power with non electric bike weight. For me I am sure I can hussle a FF along enough to make it super fun and then have the whole enhanced feel of power and range etc. As you say I am sure the two types of bike will converge, FF get lighter, SL more power. Just preference but I wanted a different experience which is what it's all about for me. Also it depends on how deep your pockets are you can spend more and get FFs which are heading towards the heavier versions of the SL for example.